The Momentus is supposed to be one of the fastest hdd drives coming very close to the speed of a ssd. I love speed but for $60 less and 320GB more..... Is the speed difference that noticable? I'm not a hardcore anything. Just a casual gamer/media.
The momentus speeds up reads, but with only 32gb, not all of your data will be in the cache, available for a fast read.
And, when it comes time to write, you are still limited by the mechanical speed of the hard drive.
It seserves a place in a laptop where you are limited to one drive, and a ssd won't hold it all.
In a desktop, start with a ssd, and later add a hard drive for bulk storage if you need to.